Fungi

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Fungi are a kingdom of their own, neither plant nor animal, yet they shape nearly every ecosystem on land. They decompose dead wood, feed about 90 percent of plant species through root partnerships, and supply the world's antibiotics. This chapter covers how a fungus is built, how it reproduces, the four major phyla, and the roles fungi play as decomposers, mycorrhizal partners, and lichen partners. The material builds on cell biology from Biology 1 and connects forward to plant nutrition and community ecology.

Hyphae and Mycelium Structure

The body of a fungus is usually a mycelium, a network of threadlike hyphae that grows through soil, wood, or living tissue. Each hypha is a tube that lengthens only at its tip, extending outward into fresh territory as the fungus feeds. Because hyphae are thin and densely branched, a mycelium has an enormous surface area relative to its volume, and a single mycelium can sprawl across many square meters of forest floor. The largest known organism on Earth is a mycelium: an Armillaria ostoyae specimen in Oregon spans roughly 3.7 square miles and may be thousands of years old.

Fungal cells are wrapped in walls of chitin, the same tough polymer that stiffens insect exoskeletons, rather than the cellulose of plant walls. Chitin is a nitrogen-containing polysaccharide built from repeating units of N-acetylglucosamine, and it makes hyphae strong enough to push between soil particles and even through the cell walls of living hosts. The chitin wall is one of several traits that place fungi closer to animals than to plants on the tree of life.

Hyphae come in two structural arrangements. Septate hyphae are divided into compartments by cross walls called septa, and each septum is usually perforated so cytoplasm and even nuclei can move between compartments. Coenocytic hyphae have no septa at all, so the whole tube is one continuous cytoplasm that holds many nuclei. Bread molds and other zygomycetes are typically coenocytic, while most mushrooms and sac fungi are septate. Some parasitic fungi, such as the rusts and mildews, also produce specialized hyphae called haustoria that push into host cells and siphon nutrients directly from them.

Feeding happens by absorption, never by swallowing. A hyphal tip secretes digestive enzymes onto the surrounding food, the enzymes break large molecules into small ones outside the cell, and the fungus then takes the small products up across its wall and membrane. Because digestion is external, the mycelium must be woven through its food source; in effect, a fungus lives inside its meal. This strategy is so effective that the mycelia of a forest together outweigh its animals many times over.

Common Mistake: Calling fungi plants is the most common error on this topic. Fungi do not photosynthesize, their walls are made of chitin rather than cellulose, and they feed by absorption rather than by making their own food. Genetically and structurally, they are closer to animals than to plants.

ELI-10

Picture a spider web growing underground instead of above ground. The fungus is a tangled net of threads, and each thread is a hypha. The whole net is the mycelium. Because the net spreads wide, it touches lots of soil and can soak up food across a huge area.

Sexual and Asexual Reproduction

Fungi reproduce both asexually and sexually, and most species can switch between the two routes depending on conditions. Harsh or crowded environments favor sexual reproduction, which shuffles genes; abundant food favors asexual reproduction, which floods the neighborhood with identical copies.

Asexual reproduction is the fast route. It happens by fragmentation, by budding, or by spores made through mitosis. Fragmentation is exactly what it sounds like: a piece of mycelium breaks off and grows into a new mycelium, the way a weed regrows from a broken stem. Budding, typical of yeasts such as Saccharomyces, forms a small outgrowth on the parent cell that pinches off as a new cell. Many molds produce enormous numbers of asexual spores instead. Bread mold Rhizopus raises upright stalks called sporangiophores tipped with round sporangia that fill with spores, while Penicillium and its relatives pinch off chains of spores called conidia from the tips of specialized hyphae. Asexual spores are genetically identical to the parent and are carried by wind, water, or animals, so they can travel far and germinate anywhere food appears.

Sexual reproduction follows three diagnostic steps. In plasmogamy, the cytoplasm of two compatible hyphae fuses while their nuclei remain separate, which often produces a dikaryotic cell or mycelium holding two nuclei per compartment (n + n). Later, in karyogamy, the paired nuclei fuse into a single diploid nucleus (2n). Finally meiosis divides that diploid nucleus into haploid spores, and each spore can germinate into a new haploid mycelium. In mushrooms the long-lived, visible mycelium is dikaryotic, and the mushroom itself is only a fruiting body built to make and release spores.

1. Two compatible hyphae fuse cytoplasm in plasmogamy.
2. Paired nuclei stay separate, forming a dikaryotic mycelium.
3. Karyogamy fuses the paired nuclei into one diploid nucleus.
4. Meiosis produces haploid spores.
5. Each spore germinates into a new haploid mycelium.

Common Mistake: Fungal spores are not seeds. A seed contains a fertilized embryo and food reserves, while a fungal spore is a single cell that germinates directly into a new hypha. Spores produced asexually are exact copies of the parent, not offspring of two parents.

ELI-10

Imagine a garden where one plant can sprout new copies of itself from a fallen leaf. That is asexual reproduction, and a fungus can do it with a single broken-off piece. Sexual reproduction is like two plants swapping flowers so their seeds mix. Mixing gives the next generation new combinations of traits. Most fungi can use both ways.

Major Phyla

The roughly one hundred thousand described species of fungi are traditionally organized into four major phyla, distinguished mainly by the details of sexual spore formation. Molecular data have rearranged some branches and added a few small groups, but these four remain the core of every course on fungal diversity.

Chytridiomycota, the chytrids, is the most ancestral phylum and the only one whose spores swim. Its motile spores and gametes carry a single posterior flagellum, so chytrids live in water or damp soil, where they decompose pollen, keratin, and chitin. They are also the cause of chytridiomycosis, the disease that has driven many amphibian populations to collapse.

Zygomycota, the zygomycetes, are fast-growing molds with coenocytic hyphae, best known from the bread mold Rhizopus stolonifer that fuzzes over stale bread. Their sexual stage is a thick-walled zygosporangium in which nuclei from two mating types fuse and then divide by meiosis. When bread molds reproduce asexually, their upright sporangia are invisible without a microscope.

Ascomycota, the sac fungi, is the largest phylum, containing more than half of all named fungal species. Its sexual spores, called ascospores, form inside microscopic sacs called asci, often eight spores per ascus. Morels, truffles, most yeasts, and the Penicillium mold that yields penicillin are ascomycetes, and most of the fungal partners in lichens also belong here.

Basidiomycota, the club fungi, produce sexual spores called basidiospores on club-shaped cells called basidia, which line the gills of the familiar mushroom cap. The mushroom is only the fruiting body; the bulk of the organism is the dikaryotic mycelium underground. Rusts, smuts, puffballs, and shelf fungi are basidiomycetes, and this phylum contains the champion wood-rotters.

ELI-10

Think of four families that all live in the same apartment building. They share walls and plumbing, but each family has its own way of making children. One family makes babies in water, one in a round pod, one in a tiny sac, and one on a little club. A mushroom is the club family's house for making spores. Every family is still a fungus.

Decomposers and Mycorrhizae and Lichens

Decomposers, called saprotrophs, feed on dead organic matter, and fungi are the planet's premier decomposers of wood. Cellulose is already hard to break down, but lignin, the stiff glue that holds wood together, is harder still, and fungi are the only organisms that digest lignin efficiently. White-rot fungi remove both lignin and cellulose, leaving pale fibrous wood, while brown-rot fungi attack cellulose and leave the brown lignin behind. By shredding dead trees and returning carbon, nitrogen, and phosphorus to the soil, fungi keep forests from burying themselves in their own litter.

Mycorrhizae are mutualistic associations between fungal hyphae and plant roots, and about 90 percent of plant species form them. The fungal network spreads through soil far beyond the reach of root hairs and delivers water and mineral nutrients, especially phosphorus, to the plant; the plant pays the fungus with sugars from photosynthesis. Two major forms exist. Ectomycorrhizae wrap the root in a fungal sheath and send hyphae between root cells without entering them, and they dominate in forests of pines, oaks, and beeches. Arbuscular mycorrhizae push hyphae inside root cells, where the fungus forms branched exchange structures called arbuscules; these associate with most crop plants and grasses. Some orchid seeds cannot even germinate without their mycorrhizal partner.

1. Fungal hyphae grow toward young roots and attach to their surface.
2. The fungus forms a sheath around the root or enters its cells.
3. The plant passes sugars to the fungus through the root.
4. The fungus passes water and minerals from distant soil to the plant.
5. Both partners grow better together than either does alone.

Lichens are composite organisms: a fungus, usually an ascomycete, living in symbiosis with a photosynthetic photobiont, which is an alga or a cyanobacterium. The fungal partner weaves a protective body called a thallus that holds water and anchors the pair to bare rock, bark, or soil, while the photobiont performs photosynthesis and shares the sugars. Cyanobacterial photobionts can also fix nitrogen, fertilizing the thallus and the ground beneath it. The partnership survives on bare surfaces where neither partner could live alone, which is why lichens are the first colonists of fresh lava and glacier moraines. Lichens absorb everything from the air without any filter, so they are sensitive bioindicators of air pollution, and three growth forms are commonly distinguished: crustose lichens pressed flat against rock, foliose lichens with leafy lobes, and fruticose lichens that stand up like tiny shrubs.

Common Mistake: A lichen is not one organism, and it is not a moss or a plant. It is two organisms living together, a fungus plus an alga or a cyanobacterium. The word "lichen" names the whole partnership, not a single species.

ELI-10

A banana left on the counter turns brown and mushy because tiny eaters are digesting it on the outside. Fungi are the clean-up crew of nature. They squirt digestive juice onto dead leaves and wood, then drink the soup that results. Without fungi, forests would be buried in fallen trunks.

ELI-10

A tree and a mushroom can be roommates who help each other. The fungus threads wrap around the tree's roots like a cozy blanket that also reaches far into the dirt. The blanket hands water and minerals to the tree, and the tree pays the fungus with sugar it made from sunlight. Nearly all plants have such a roommate.

ELI-10

A lichen is like a tiny floating island with two residents. One resident is a fungus that builds a safe house. The other resident is an alga that cooks food using sunlight inside the house. The house protects the cook, and the cook feeds the house. Together they survive on bare rocks where neither could live alone.

High-Yield:

• Fungal cell walls contain chitin, not cellulose.
• The four major phyla are Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota.
• Fungi are the main decomposers of wood and the only organisms that break down lignin efficiently.
• About 90 percent of plant species form mycorrhizae.
• A lichen is a fungus plus an alga or a cyanobacterium, not a single organism.

Quick Review

• The fungal body is a mycelium of threadlike hyphae with chitin walls; hyphae are either septate or coenocytic.
• Fungi feed by absorbing externally digested molecules, never by swallowing food.
• Asexual reproduction uses fragmentation, budding, and mitotic spores; sexual reproduction proceeds through plasmogamy, karyogamy, and meiosis.
• Chytridiomycota make flagellated spores, Zygomycota make zygosporangia, Ascomycota make ascospores in asci, and Basidiomycota make basidiospores on basidia.
• Saprotrophic fungi are the main decomposers of wood because they digest lignin, which few other organisms can break down.
• About 90 percent of plant species rely on mycorrhizal fungi for water and minerals, especially phosphorus.
• Lichens are a fungus living with an alga or a cyanobacterium, and they pioneer bare rock.
• Spores are single cells that germinate directly, not seeds containing an embryo.

Key terms

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Important formulas or processes

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Common mistakes

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